Place the bacon in a medium nonstick skillet and fry over medium heat, stirring occasionally, until crisp, 8 to 10 minutes.

While the bacon cooks, in a large bowl, whisk together the vinegar, honey, mustard, salt, and pepper. Set aside.

Using a slotted spoon, transfer the cooked bacon to a paper towel–lined plate.

Pour the bacon fat into a heatproof bowl, then return 4 tablespoons of the bacon fat to the skillet. Add the shallots to the skillet and cook over low heat, stirring frequently, until softened, 1 to 2 minutes.

Add the bacon fat and shallots from the skillet to the vinegar mixture and whisk to combine.

Add the spinach and mushrooms to the dressing and toss to coat evenly.

Taste and adjust the seasoning with salt and pepper, if necessary.

Divide the salad between 4 plates or bowls; evenly divide the egg and cooked bacon among them.

Serve immediately.